Cancer is a state of cellular growth occurring when some normal cells become abnormal and continue to grow abnormally, which, in the many forms we know it, is a disease of civilization, and is practically unknown among primitive people properly nourished on a simple natural diet and lived by a simply lifestyle. Nowadays, cancer is becoming the major cause of deaths in many parts of the world. Early detection has been demonstrated to be the most effective strategy to reduce cancer mortality. Thanks to the development of modern technology, a plurality of non-invasive diagnostic examination devices, that involve the acquisition of physiologic images based on the detection of molecular biological processes, have been developed and thus the probability of early detection has been greatly increased. Among those diagnostic examination devices, the usefulness of positron emission tomography (PET) imaging has recently become accepted, not only in research, but also in clinical fields.
PET is a nuclear medicine medical imaging technique which produces a three dimensional image in the body, in which a short-lived radioactive tracer isotope which decays by emitting a positron (β+), chemically incorporated into a metabolically active molecule, such as glucose or amino-acid, etc., is injected into the blood circulation of a living subject by intravenous injection. There is a waiting period while the metabolically active molecule becomes concentrated in tissues of interest, then the subject is placed in the imaging scanner. The short-lived isotope decays, emitting a positron. After travelling up to a few millimeters, the positron annihilates with an electron, producing a pair of annihilation photons, which is similar to gamma (γ) rays of 511 keV moving in opposite directions, i.e. being emitted 180° apart. These are detected when they reach a scintillator material in the scanning device, creating a burst of light which is detected by photomultiplier tubes where it is converted and amplifuied into digital signals. The technique depends on simultaneous or coincident detection of the pair of photons. The digital signals are reconstructed by processes of binning, rebinning and image-reconstructing so as to plot a map of the distribution of the gamma rays.
As a conventional image reconstruction method shown in FIG. 1 where we have an annihilation referred as an event, the gamma ray is traveling at opposite directions and detected in detectors and a line of response (LOR) 12 is drawn in between those two detectors 10, 11, moreover, associated with that line of response 12 are two paired scintillator crystals respectively arranged on the two detectors 10, 11. By which, as an annihilation event of the LOR 12 is detected by the two detectors 10, 11, the counting of the LOR 12 is increased by one. It is known that, during a specific scanning period, there may be a plurality of annihilation events relating to the LOR 12 as the counting thereof accumulated therewith, and the point of interaction or annihilation can be any point on the LOR 12. However, the conventional image reconstruction method of FIG. 1 is based on an assumption that any positron annihilation is occurred right at the middle of its corresponding LOR and can be determined at the intersection of the focal place, such as the A, B and C planes of FIG. 1, with the LOR.
The abovementioned focal place tomography is disclosed by H. Uchida et al, in “A compact planar positron imaging system,” Nucl. Instrum. Methods Phys Res. A, vol. 516, pp. 564-574, 2004. The focal place tomography can only reconstruct a planar image with respect to the coordinate (X, Y) data on a focal plane, but the data of an annihilation relating the a third dimension represented by the Z direction of FIG. 1 is missed. In addition, a nuclear imaging using variable weighting is disclosed in U.S. Pat. No. 5,793,045, which use a weight processor to weight each coincidence event based on the energy of the detected gamma rays. For instance, a true event occurs when a gamma ray is detected without having been scattered. Not having been scattered, these gamma rays are characterized by energies in the region of the primary photopeak of the particular radiopharmaceutical in use (e.g. 511 keV for gamma rays generated by positron annihilation). Detected events in the upper portion of the primary photopeak are particularly unlikely to have been scattered. The positions of these events can therefore be determined with an especially high degree of confidence. As a result, true events contribute positively to image quality. Nevertheless, the image reconstruction method disclosed in U.S. Pat. No. 5,793,045 is still a planar image reconstruction method that it can not determine effectively the exact position of each annihilation. Moreover, the contrast of the image reconstructed thereby is poor while there are a plurality of positron annihilations in an object-to-be-detected.
In the conventional method disclosed in U.S. Pat. No. 6,804,325, entitled “Method for positron emission mammography image reconstruction”, the image reconstruction is implemented either by backprojection image reconstruction or by iterative image reconstruction utilizing maximum likelihood expectation maximization (MLEM). However, in the backprojection image reconstruction provided in the forgoing U.S. patent, the probability of each positron annihilation is assumed to be the same.
Therefore, it is in need of an image reconstruction method for achieving three-dimensional imaging with improved contrast and better tumor-detection accuracy on an objected to be imaged, that it is cost-saving and can perform a scan with less time comparing to those of conventional planar imaging systems.